B.G 2.21
वेदाविनाशिनं नित्यं य एनमजमव्ययम् । कथं स पुरुषः पार्थ कं घातयति हन्ति कम् ॥ २-२१॥
vēdāvināśinaṁ nityaṁ ya ēnamajamavyayam । kathaṁ sa puruṣaḥ pārtha kaṁ ghātayati hanti kam ॥ 2-21॥
O Arjuna, how can the Jīva who knows thus the Indestructible Lord to be (the doer), ever existing, unborn and unchangeable, think of being killed or hurting another?
Gīta Bhāshya 2.21
The words 'avinashina' and 'nitya' refer to the Lord. Word 'nashtha' is commonly used to address Jīva, as it means tainted with defect. The word 'Vēda' refers to the imparted knowledge till previous verse, that the Lord is the doer of all acts and Jīva is not an independent agent.
How can one who understands thus (Jîva to be eternal and dependent) think of killing or causing another to kill?
"avinâshina" - means one who does not have a destruction of the form; and "nitya" - means one who does not have a destruction of original nature;
or
"avinâshina" - means one who is untouched by defects, and "nitya" - means one who exists always.
Above said is common wisdom.
As Jîva is tainted with defects, the word 'nashta' is used to address it.
Gīta Tātparya 2.21
'avinashina' indicates lord without destruction of his body, 'nitya' indicates non-variance in his essential nature. The agency must always primarily be attributed to the Lord and only the ignorant attribute it to themselves.
The word 'avinashinam' indicates one who is devoid of defects such as the destruction of the body; 'nityam' indicates one's own essence; 'enam' is a reference to the supreme lord.
"The notion of agency is nothing but freedom to do. It exists in Lord Hari alone. It is present in Him always, without destruction. How can one with this understanding think of himself as a doer?" - thus states Parama sruti
If not interpreted this way, the words 'avinashina' and 'nitya' providing same meaning results in redundancy error.
